This invention relates generally to screw extruders and more particularly to determining torsional deformations in screw extruders.
Thermoplastic resins are commonly formed using extrusion molding machines typically referred to as extruders. Both single screw and multi-screw extruders are known. For instance, a twin screw extruder includes a pair of intermeshing screws rotatively mounted within a close fitting casing or barrel. Raw material, typically in the form of powder or pellets, is fed into the interior of the barrel and is moved through the barrel by the rotating screws. The mechanical action of the screws, along with any heat that may be added, melts and mixes the raw materials. The heated and compressed material is forced out of a die at the discharge end of the barrel and assumes the desired shape.
Along the length of each screw, there are many different material regimesxe2x80x94solids, voids and liquids of varying viscosity. Because they are not perfectly rigid bodies, the extruder screws act like long torsion springs when encountering these varying material regimes. That is, the screws will experience angular twist or torsional deformation.
A major cause of customer rejection of extruded plastics is variations in viscosity of the finished product. Rejected material increases production costs and leads to dissatisfied customers. There are many factors that can effect viscosity, including the quality of the raw materials used, the amount of heated applied and the rotational speed of the screws. Monitoring the extrusion screw torque would allow better control of the process such that variations in viscosity could be reduced. However, merely measuring the torque at the drive motor will not provide optimal control of the extrusion process because of the torsional deformations along the length of the screws due to the above mentioned variations in the material regimes that the screws encounter.
Accordingly, it would be desirable to be able to continuously measure rotating extruder screw torsional deformations along the length of the screw. Knowledge of the varying torsional deformations allows for better control of the extrusion process leading to reduced viscosity variations in the finished product. Reducing variations in the finished product will increase yields, thereby reducing overall production costs.
The above-mentioned need is met by the present invention, which provides an extruder having a barrel including a die attached to one end and at least one material inlet with a feeder for providing material to the material inlet. At least one screw is rotatively mounted in the barrel, and a motor is provided for driving the screw. A plurality of sensors is mounted in the barrel for sensing passage of screw thread edges as the screw rotates. A controller for controlling operation of the extruder receives signals from the sensors and determines local torsional deformations of the screw based on the signals. The controller slows down the extruder if any one of the local torsional deformations falls outside of an optimal range.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.